1. Technical Field
The present inventions relate to an integrated circuit chip and, more particularly, to a chip that uses at least one external feedback signal to select a transmitting voltage.
2. Background Art
Many chips are designed to provide output transmit signals at voltages that are chosen for worst case situations. Since worst case situations usual do not occur, these chips use more power than is needed.
Some chips are designed to operate at more than one voltage. For example, a mobile computer chip may operate at one voltage when its power is supplied from an external source and a lesser voltage when the power is supplied from a battery. However, the two voltages are pre-selected.
Loopback refers to a process wherein a first chip transmits signals to a second chip and the second chip retransmits the signals back to the first chip. Loopback is used in analog and digital validation, test, and debug.
Various techniques have been developed for chips to communicate with each other. The techniques include those that have been standardized and those that have not been standardized. An example of standardized techniques include versions of the Peripheral Chip Interconnect (PCI) Local Bus Specification, such as a PCI Local Bus Specification rev. 2.2, dated Dec. 18, 1998, by the PCI Special Interest Group (SIG). A PCI Express specification, formerly known as 3GIO (3rd generation input output), is being defined by the PCI SIG. A PCI Express Base Specification Revision 1.0, Jul. 22, 2002, has been released and is available with the payment of a fee. An Errata dated Oct. 7, 2003 has been published.
Signals are often applied differentially on two interconnects. The term “common mode” refers to the average voltage on the interconnects. Systems have used capacitors in series with interconnects to pass the difference between signals on the two interconnects, but not the common mode under ordinary operation. The 8b/10b code is a commonly used code in serial links.